The present invention relates to a thermal conductive material applied between an exothermic portion and a cooling portion in electric and electronic apparatus etc. More specifically, the present invention relates to a highly thermal conductive grease composition comprising an inorganic powder and a cooling device applied with same.
Efficiency of a cooling device used for removing heat generated by an integrated circuit element greatly depends upon a thermal conductivity, i.e. a thermal resistance, of a thermal conductive grease which is called also a thermal conductive compound and which is filled in a contact surface between an exothermic surface and a radiating portion in the integrated circuit element. As a material used for the thermal conductive grease composition for the integrated circuit element, there is used a material that is possessed of an electrical insulation and that is capable of quickly removing the generated heat through a radiating portion, because the grease composition is brought into indirect or direct contact with both the integrated circuit element and the radiating surface.
As the thermal conductive grease, there are so far known those comprising a base oil including a silicone oil such as polydimethylsiloxane and polymethylphenysiloxane and a heat transfer substance including a powder such as aluminum nitride, silica, alumina, metal silicon, boron nitride and zinc oxide (cf. JP-A 51-55870, JP-B 52-33272, JP-A 54-116055, JP-A 55-45770, JP-A 61-157587, JP-A 2-153995, JP-A 2-212556, JP-A 3-14873, JP-A 3-162493, Japanese Patent No. 2925721, Japanese Patent No. 2938429, and JP-A 2000-109373).
Japanese Patent No. 2930298 discloses a thermal conductive grease composition comprising an aluminum nitride powder which is surface-treated with an organosilane and/or a partially hydrolyzed condensate thereof and a base oil of a liquid hydrocarbon oil or a fluorohydrocarbon oil. However, the thermal conductivity thereof is about 2.3 W/mxc2x7K, which is not satisfactory.
Japanese Patent No. 2938429 discloses a thermal conductive silicone composition comprising a silicone oil and thermal conductive inorganic fillers different from each other in each Moh""s hardness. However, the thermal conductivity thereof is also from 2.72 to 3.97 W/mxc2x7K, which is not sufficiently satisfactory.
For the purpose of preventing isolation and bleeding of the silicon oil, JP-B 57-36302 discloses a thixotropic thermal conductive material prepared using a silica fiber, a dendrite like zinc oxide, a laminal aluminum nitride and a laminal boron nitride.
Further, there are disclosed a thermal conductive grease comprising perfluoro polyether as the base oil in JP-A 63-251455 and JP-A 3-106996, which is proposed mainly to improve a contact default, some grease blended with a urea compound in JP-A 4-117482, some grease added with a fluoro-surfactant in JP-A 63-57693 and JP-A 4-239597, which are proposed to prevent isolation and bleeding of the oil, and some grease prepared using a fluorine compound having a polyfluoroalkyl group and at least one oxyalkylene group in JP-A 10-140173. However, the thermal conductivity thereof is about 2.3 W/mxc2x7K, which is not satisfactory from a viewpoint of heat removal.
Japanese Patent No. 2938428 discloses some grease comprising a liquid hydrocarbon oil and/or a fluorohydrocarbon oil as the base oil and a combination of a specific thermal conductive inorganic filler having a thermal conductivity of not less than 100 W/mxc2x7K and another specific thermal conductive inorganic filler having a thermal conductivity of not less than 20 W/mxc2x7K, which are proposed to further improve a dispense property and a high thermal conductivity. The thermal conductivity of the grease disclosed is as remarkably good as from 2.59 to 4.02 W/mxc2x7K, and its dispense property is also good. A content of the base oil which is a liquid hydrocarbon oil and/or a fluorohydrocarbon oil used in these grease is found to be 10% by weight, which is calculated on the basis of the disclosure of Example. An oil-isolation degree measured at 150xc2x0 C. for 24 hours according to JIS-K-2220 is found to be 0% by weight in every cases. Notwithstanding, a diffusion due to bleeding of the oil is caused when the grease is subjected to a heating test of 150xc2x0 C./20 hours, in which the grease is coated on an aluminum nitride plate in a circular and gable roof form.
It is an object of the present invention to provide a highly thermal conductive grease composition having both a high thermal conductivity and a satisfactory dispense property. It is another object of the present invention to provide a cooling device applied with same.
The present invention provides a highly thermal conductive grease composition which comprises an inorganic powder and a base oil containing a mineral oil or a synthetic oil, wherein the inorganic powder is a mixture of at least two kinds of inorganic powders different from each other in each average particle size, the base oil further contains a surfactant in an amount of from 0.2 to 2.0% by weight based on the weight of the inorganic powder, and a content of the base oil is from 10 to 30% by volume, and a content of the inorganic powder is from 70 to 90% by volume.
The present invention further provides a cooling device provided with an electric or electronic component assembled into an apparatus and a cooling body put on a surface of said component, in which the highly thermal conductive grease composition as above intervenes between said cooling body and a surface of an exothermic body in said component.